Digital printing is known as a technique, and it is widely used, among others, in colour printing, copying machines and printers. The application EP 629930 discloses a digital printing technique, with which a multi-colour printing is achievable on one side or both sides of a moving paper web. The different shares of the printing are produced in successive printing stations along the path of the web, the printing stations being arranged to operate in a synchronised way. Each station comprises a rotating drum, with an accumulator installed on its periphery, producing a uniform electric charge to the surface of the drum. On the periphery of the drum, the accumulator is followed by a printhead, such as a laser scanner, which forms a latent image to the surface of the drum by selectively changing the charge of the drum surface, the latent image being then developed in a developing station, in which opposite signed toner particles are brought to the surface of the drum in accordance with the image. After this, the surface of the drum is brought into contact with the paper web led past it for transferring the toner particles forming the image to the surface of the web. For this purpose, in the point of contact of the drum and the web, a corona transfer assembly has been installed on the opposite side of the web, the electric current directed through which forming an electric field, which draws the electrically charged toner particles from the surface of the drum to the surface of the paper web. Immediately adjacent to the corona transfer assembly there is installed an alternating-current corona apparatus, which eliminates the charges of the web and allows it to separate from the surface of the drum. The surface of the drum is then pre-charged by the corona apparatus and cleaned from the toner particles possibly remaining on it, after which the surface is ready for a new printing cycle, which may as well be identical with the previous cycle as be different from it.
Black-and-white printing can be produced on the one side of the paper in one single printing station using black toner in a way disclosed above. In multi-colour printing, the different toners are brought to the paper in several successive printing stations which operate with different colours, adding them to the printing to be generated to the moving web one at a time. The printing of both sides of the paper can still be achieved by placing printing stations of the type disclosed above to both sides of the moving paper web.
After the printing consisting of one or more toners is produced onto the paper as disclosed above, the printing is fixed in a fixing station located on the path of the web. The fixing is performed using infrared radiators, which heat the surface of the web so that polymeric toner particles melt fast to the paper. Finally, the finished printed web is cut to sheets, which are piled or stitched, according to the need of any given time.
An essentially corresponding technique is applied to copying machines and printers, in which the printing base is formed of individual sheets, instead of a continuous web. In addition to paper sheets, also plastic films can be used as a base in copying machines.
In the printed patent specification U.S. Pat. No. 5,741,572, there is disclosed a paper intended to be printed electrophotographically, which is coated with ionomer (Surlyn 1605) or a mixture of ionomer and some other polymer. For the toner to be fixed with the help of heat, the specification likewise suggests ionomer to be used. According to the specification, ionomer resins do not tend to spread, due to which the print lasts well during and after thermal fixing.
The drawback of digital printing technique in the printing of board webs has been the typically more irregular surface of the boards, which causes a printing result of poor quality. Boards have been printed using conventional printing techniques, such as offset printing. Especially polymer coated packaging boards used for packages and disposable dishes can have been printed before the coating phase, in which case coating layers consisting of colourless or transparent polymers have been extruded onto the board surface provided with printings, or offset printing can have been performed onto a board pre-coated with polymer.